Viral haemorrhagic disease of rabbits in the People's Republic of China:

Rev. sci. tech. Off. int. Epiz., 1991, 10 (2), 393-408
Viral haemorrhagic disease of rabbits
in the People's Republic of China:
epidemiology and virus characterisation
WEI-YAN XU *
Summary: Viral haemorrhagic disease (VHD) is a new and severe infectious
disease of rabbits, with a high rate of morbidity and mortality. The disease occurs
throughout the year, affecting only adult rabbits and not other domestic animals,
fowls or laboratory rodents. The transmission is horizontal, by direct or indirect
contact, and through all routes. There is no evidence of congenital infection
or biological vectors.
The causative agent, viral haemorrhagic disease virus (VHDV), is present
in all tissues, excretions and secretions. It is an icosahedral and nonenveloped
parvo-like virus. The genome, as determined by classical methods, high
performance chromatography and in vitro synthesis of double-stranded DNA,
is linear, single-stranded DNA. VHDV can agglutinate human erythrocytes at
very high titres, irrespective of blood groups, and has a stable reaction to many
physical and chemical factors. VHDV has been adapted to grow on rabbit kidney
cell strain (DJRK) culture and to produce cytopathic effect (CPE). Inactivated
cell culture can protect inoculated rabbits against virulent VHDV.
The disease is now effectively controlled in the People's Republic of China,
but has not yet been completely eradicated.
KEYWORDS: Cell culture - Epidemiology - Genome cloning Haemagglutination - Haemorrhagic disease - High performance
chromatography - Parvovirus - Single-stranded DNA - Transmission - Viral
haemorrhagic disease virus of rabbits.
INTRODUCTION
Viral haemorrhagic disease (VHD) is a new, peracute and highly communicable
disease, affecting adult rabbits only. It is characterised by sudden death, haemorrhages
in various tissues, bloody exudates in the trachea and a r o u n d the nostrils, and by
haemagglutination (HA) of h u m a n red blood cells at a very high titre. The aetiological
agent is a unique virus which was not reported in worldwide literature before 1984.
Owing to contradictory results reported on the nature of the genome, n o consensus
has been reached concerning its classification as a virus. The author bases his
assumption that it is a parvo-like virus on the type of genomic nucleic acid involved
and on cross-reaction with certain other parvoviruses.
* Department of Veterinary Science, Nanjing Agricultural University, 210014 Nanjing, Jiangsu
Province, People's Republic of China.
394
C H A R A C T E R I S A T I O N OF
VIRAL HAEMORRHAGIC DISEASE VIRUS
Morphology
The morphology of viral haemorrhagic disease virus (VHDV) was examined under
the electron microscope by negative staining with phosphotungstic acid after a sample
from the liver emulsion of an infected rabbit had been concentrated and purified
by differential centrifugation and polyethylene glycol (PEG) sedimentation, or further
by extraction with chloroform and other organic solvents. V H D V is approximately
32-34 nm in diameter and varies slightly in different preparations (2, 3, 4, 6, 9, 13,
16, 18, 20, 26, 29). The virion is icosahedral a n d nonenveloped a n d has a core of
18-20 nm (4, 9, 13). The capsid is composed of 32 capsomeres (9, 18) (Fig. 1). The
virions in thin sections of rabbit liver and kidney cell strain (DJRK) from infected
cell culture are slightly smaller and are found in the cell nuclei and around the nuclear
m e m b r a n e (8, 14, 17).
FIG.
1
Viral haemorrhagic disease virus, negative staining,
x 216,000
Sedimentation coefficient and buoyant density
The purified virus preparation is layered on a continuous sucrose gradient from
20 to 6 0 % and centrifuged at 18,400 r p m . The sedimentation coefficient of V H D V ,
determined by ultraviolet scanning, is 162S. The purified virus preparation layered
395
on 3 5 % CsCl is centrifuged at 40,000 r p m for 24 h. The virus band is collected
and examined with A b b é ' s refractometer. The buoyant density of V H D V is
1.36-1.38 g / c m (2, 10).
3
Nucleic acid
Because the virus is difficult to grow on cell culture and to purify without
contamination of host cell components, there was much disagreement among Chinese
scientists between 1984 and 1986 over the nature of the nucleic acid of V H D V . The
dispute subsided after highly purified preparations and modern techniques,
complementing classical methods, had been used.
Type
The partially purified virus preparation is further treated with ribonuclease (RNase)
and deoxyribonuclease (DNase) to degrade the contaminating nucleic acids of host
cells adsorbed on the virion capsid. The highly purified virus preparation is examined
by diphenylamine and orcinol reactions and the nuclease degradation test (2, 18, 20).
The electrophoretically pure nucleic acid is also analysed by high performance liquid
chromatography for its base composition (20, 21). The results are summarised in
Table I and Figure 2.
TABLE I
Identification
of the type of nucleic acid of
VHDV
Procedure
VHDV-NA
Yeast RNA
Thymus DNA
Diphenyl amine
Orcinol
DNase 1
RNase A
Base composition
Blue
No change
Degraded
Resistant
No change
Dark green
Resistant
Degraded
Blue
Light green
Degraded
Resistant
C, G, A , T
C, G, A , U
C, G, A, T
Since V H D V nucleic acid turns blue in the diphenylamine reaction and light green
in the orcinol reaction, is degraded by DNase 1 but resistant to RNase A , and is
composed of four nucleoside bases, adenine (A), thymine (T), guanine (G) and cytosine
(C), there is n o doubt that it is a D N A .
Strand
The highly purified virus preparation has been subjected to several tests. In the
melting point (Tm) test, the purified nucleic acid is dissolved in standard saline-citrate
buffer (0.15 M NaCl and 0.015 M sodium citrate, p H 7.0). The optical density (OD)
values of the solution are recorded every 5 min from 18 C to 95 °C by ultraviolet
spectrometry at a wavelength of 260 n m . The O D values increase only slightly from
0.25 to 0.28. This indicates that the V H D V nucleic acid cannot be separated by heat,
unlike that of the double-stranded (ds) D N A . In acridine orange staining the nucleic
acid band in agarose gel, after electrophoresis, gave a red flame when examined under
a high pressure mercury l a m p . In the nuclease degradation test, the nucleic acid was
degraded by Alu 1, but resistant to DNase S1 (2, 20). In high performance liquid
2
Left: VHDV nucleic acid
Middle: calf thumus DNA
Right: yeast RNA
Note the difference in the four nucleotide bases between DNA and RNA
C = cytosine
G = guanine
T = thymine
A = adenine
U = uridine
High performance liquid chromatogram of hydrolysates
of VHDV nucleic acid
FIG.
396
397
chromatography, A = 2 4 . 4 % , T = 3 1 . 9 % , C = 2 3 . 7 % and G = 2 0 . 8 % (19). Since
G is paired with C, and T with A , the mole percentages of G and C, or T and A ,
have to be approximately equal in d s D N A . Here G and C, and T and A , are unequal,
which provides indirect evidence that the nucleic acid tested is single-stranded. T h e
results are summarised in Table II.
TABLE II
Identification
of the nature of the nucleic acid strand of
Nucleic acid
of VHDV
Procedure
Control
Phage X
ssDNA
Calf thymus
dsDNA
OD value increase
at 260 nm
Green
Resistant
Degraded
Heat denaturation
No change
No change
Acridine orange
DNase SI
Alu 1
Base composition
by HPLC
Red
Degraded
Resistant
Red
Degraded
Resistant
C
G
T
A
=
=
=
=
23.7
20.8
31.9
24.4
VHDV
C
G
T
A
Not done
=
=
=
=
21.2
21.5
27.8
28.2
Shape
After elimination of the contaminating cellular nucleic acid with nucleases, the
purified virions are treated with urea to elicit the release of the nucleic acid from
the capsid. After being spread on water, stained with uranium acetate and shadowed
with platinum, the specimen is observed under the electron microscope. As shown
in Figure 3, the nucleic acid is not segmented and is linear in shape, with an average
length of 1.92 µm (1.84-2.2 µm) (28).
The results of Polyacrylamide gel electrophoresis (PAGE) and agarose gel (AGE)
also indicate that the nucleic acid of V H D V is not segmented (2).
Molecular
weight
6
The molecular weight of V H D V nucleic acid is 2.4 x 10 when compared with the
distance moved by phage M 13 single-stranded D N A after P A G E and A G E . The
molecular weight calculated from the length of nucleic acid, by multiplying by
1.2 x 10 per µm, is 2.3 X 10 . These two values are very close.
6
Terminal
6
hairpin
The evidence of classical tests and high performance chromatography indicates
that the genome of V H D V is single-stranded D N A . A m o n g vertebrate viruses, only
the genome of the Parvoviridae belongs to this type. If V H D V is a parvovirus, there
should be hairpin structures at the 3 ' - and 5'-terminals of the nucleic acid chain. To
examine this, the V H D V nucleic acid is extracted from the highly purified viral
preparation and reacted with Escherichia coli D N A polymerase 1 Klenow large
398
FIG. 3
Electron micrograph of D N A of viral haemorrhagic disease virus,
x 20,000
fragment. Double-stranded D N A is synthesised in vitro and then integrated into the
Bluescript plasmid. The recombinant plasmids collected from transformed E. coli
are identified by agarose electrophoresis and restriction enzyme cleavage. They could
hybridise with V H D V nucleic acid by dot blot hybridisation after being labelled by
nick translation (24). The evidence indicates that there is a hairpin at the 3'-terminal
of V H D V nucleic acid.
Polypeptides
T h e emulsion of infected liver was purified by a process of repeated freezing and
thawing, chloroform extraction, P E G sedimentation and differential centrifugation.
The partially purified V H D V preparation was chromatographed on a Sepharose 4B
column. Three fractions were eluted. The purified and concentrated V H D V was in
fraction two when identified by H A and A G E . This fraction was analysed for viral
polypeptides with sodium dodecyl sulphate (SDS)-PAGE. Four polypeptides were
detected; their respective molecular weights, as compared with those of marker
proteins, were as follows: VP1 = 58.3-61.1 kDa, V P 2 = 54.7 kDa, VP3 = 51-53 kDa
and VP4 = 25.9 kDa. When the electrophoretic gel was scanned with a densitometer,
the average component percentages, calculated from the 4-test integral area, were:
VP1 = 54.7 ± 1.2%, V P 2 = 17.7 ± 0 . 6 % , VP3 = 22.7 ± 1.1% and V P 4 = 4.9
± 0 . 1 % (7, 11).
In Western blotting, the polypeptides were transferred from the electrophoretic
gel onto nitrocellulose filters and reacted with:
a) V H D antiserum and peroxidase-labelled staphylococcal protein A , or
399
b) monoclonal antibodies from several hybridoma cell strains and peroxidaselabelled antimouse IgG goat serum.
All four polypeptides were detected by V H D antiserum and three monoclonal
antibodies (25, 27) (Table III). It is suggested that polypeptides V P 1 , V P 2 , V P 3 and
VP4 share c o m m o n epitopes, similar to those found in Aleutian mink disease virus.
TABLE III
Immunological
reactions
VHD V specific monoclonal antibodies and VHD V
according to Western
blotting
between
Name of MAb
VHDV polypeptides
VP2
VP3
VP1
AA8-1
CF2-1
CG4-1
CH3-1
RM-14
RM-17
+
+
+
-
-
-
+
+
•+
+
-
polypeptides,
VP 4
+
-
+
+
+
+
-
• -
-
-
-
-
Haemagglutination
Spectrum
Various tissues collected from infected rabbits immediately before and after death
can agglutinate h u m a n red blood cells at very high titres. N o obvious difference is
observed among blood groups A, B, A B and O. Erythrocytes of the chicken, goose
and sheep seem to be agglutinated at low titres (10 x 2 - 10 X 2 ), while those of other
animals, including the cow, goat, pig, rabbit, rat, mouse, guinea pig, duck and quail,
cannot be agglutinated. H A is inhibited by specific antiserum.
2
4
Distribution
The virus exists in all tissues and body fluids of infected animals. The H A titres
in the liver, spleen and serum can reach 1 0 x 2 - 1 0 x 2 , much higher than those
of other tissues (Fig. 4). There is some evidence that the H A titre is related to the
content of blood in the tissue.
1 0
Physical
and chemical
1 8
factors
H A is not significantly affected at different temperatures (4°C-37°C). There is
no strict p H value requirement; the H A titre, optimal at p H 6.0, is basically the same
between p H 4 and 7.2. When the agglutinated red blood cells (RBC) are suspended
in borate buffer at p H 9.2 and incubated at 22°C-37°C for 30 min, the viral particles
are released. The released RBC and virus are able to reagglutinate when the p H value
lowers. T w o percent NaCl in 0.01 M phosphate buffer is usually used as a diluent
of RBC in the H A test; this can enhance the H A titre to a value 1-2 times higher
than in 0 . 8 5 % NaCl (15, 23).
HA
titre
n
(10x2 )
400
FIG. 4
H A titres of various infected tissues
Li = liver
S = spleen
H = heart
M = muscle
Br= brain
Bl = blood
Lu=tung
K = kidney
Ly= lymph node
The H A activity can be destroyed by 0 . 5 % trypsin, 2 % sodium borohydride, or
2 % chloramine T, but not by receptor destroying enzyme (RDE), sodium
metabisulphite, potassium periodate, chloroform, ether or formaldehyde. The H A
activity is rather stable to heat. When the infected tissue is kept at 4 ° C and 15°C
for three m o n t h s , the H A titres decrease by 2 - 2 and 2 - 2 , respectively. If it is kept
in a waterbath at 56°C for 24 h, the titres decrease by 2 - 2 (23).
2
4
4
5
4
Receptors
6
on RBC
The viral haemagglutinin receptors on the surface of h u m a n RBC are sensitive
to chloroform and ether, and stable to trypsin, potassium periodate, R D E an
401
sodium borohydride. They are highly resistant to heat, unaffected after heating at
95°C for 15 min. The RBC fixed with 1% K M n 0 remain agglutinable by the virus
at 4 ° C , with a clear and stable agglutination pattern. Comparative study indicates
that H A results with either fixed or fresh RBC correlate well (22).
4
Resistance
The 10% liver emulsion of infected rabbits was centrifuged at 10,000 x g for
30 min. The supernatant was collected and treated in each of the following ways:
a) in a waterbath at 50°C for 60 min;
b) in 1.0 M M g C l solution in a waterbath at 50°C for 60 min;
2
c) in p H 3.0 H a n k s ' solution at r o o m temperature for 30 min;
d) in 2 0 % ether stored overnight at 4 ° C ;
e) in 10% chloroform, vibrating vigorously at r o o m temperature for 10 min;
f) control, without any treatment.
Samples of each preparation were inoculated peritoneally into six or seven
susceptible rabbits. Rabbits of the six groups all became infected and died within
36 h; however, one rabbit in G r o u p 3 (preparation c) survived, as did another in
Group 4 (preparation d) (4).
Alternately, the supernatant of the liver emulsion was mixed with an equal volume
of 2 % N a O H or 2 % formalin and was incubated at 37°C for 1 h. Each mixture was
diluted tenfold by phosphate buffered saline (PBS) and inoculated into six susceptible
rabbits, all of which survived and remained healthy, while the control rabbits
inoculated with untreated sample died within 36 h (20).
The virus is very stable at low temperatures. Infected liver samples kept at - 5°C
for 413 days or - 2 0 ° C for 560 days did not have significant drops in their infective
titres. Those kept at - 7 0 ° C for 4.5 years after lyophilisation remained infectious
without any loss (16). F o r safety purposes rabbit sheds, including the cages, floors,
walls and utensils, should be cleaned and thoroughly disinfected with 10% bleaching
powder or 3 % formalin followed by 2 % N a O H , and kept empty for two weeks in
summer, or for two m o n t h s in colder seasons.
Relationship
to some
parvoviruses
In indirect E L I S A , when purified V H D V is coated on polystyrene microtitration
plate wells and treated with V H D V reference antiserum and peroxidase-conjugated
staphylococcal protein A , or with V H D V monoclonal antibodies and peroxidaseconjugated antimouse IgG goat serum, the titre of V H D V is 1:6400. The results have
been compared with those of the canine (CPV), feline (FPV), mink (MPV), pig (PPV),
goose (GPV) and mouse (MVM) parvoviruses. When VHDV-antiserum is used, the
ELISA titres of these viruses are 1:160, 1:80,1:40, 1:80, 1:160 and 1:160, respectively.
When purified G P V is coated on microtitration plate wells and reacted with antiserum
to G P V or V H D V , the respective titres are 1:12800 and 1:320. When purified M V M
is used as antigen and reacted with antisera to M V M and V H D V , the titres are 1:9600
and 1:640, respectively. If the coated antigen is P P V and is reacted with antisera to
P P V and V H D V , the respective titres are 1:12800 and 1:320. If the coated antigen
402
is M E V and is reacted with antisera to M E V and V H D V , the titres are, respectively,
1:5120 and 1:40. The results indicate that V H D V may, to some extent, be related
in antigenicity to M V M , P P V and G P V .
Cell culture
VHDV is very difficult to grow in cell culture. Since 1984, the author has inoculated
emulsions of liver, spleen, lung and serum collected from experimentally infected rabbits
onto various cell cultures, including primary rabbit cells (kidney, liver, lung and testis)
and cell lines (PK-15, BHK-21, MA-104, IBRS-2, HeLa and VERO). Blind passages
were made and every passage was checked for H A and cytopathic effect (CPE). None
of them manifested any evidence of growth. The author also modified the method
of inoculation; for example, by inoculating just after adsorption of cells on a glass
surface, inoculating at phase SI of cell division, treating the sample with trypsin and
adding trypsin to the cell medium, substituting rabbit serum for calf serum, and using
cells together with culture fluid in passage. None of the attempts succeeded. In 1988,
a rabbit kidney cell strain, designated DJRK, was used. Regular C P E was observed
at the third passage; this was characterised by the rounding and aggregating of cells
which finally detached from the glass surface (Fig. 5). Thread-like intercellular bridges
were also observed (8). Fluorescence, principally in the cytoplasm around the cell nuclei
and, to some extent, in the nuclei at an early stage, was detected by the fluorescent
antibody technique. The cell cultures of the fifth and tenth passages were inoculated
into susceptible rabbits, all of which died within 5 to 11 days. H A appeared to titres
of 2 - 2 at 1-3 days after inoculation, 2 - 1 2 at 2-8 days after inoculation and
increased abruptly to 2 - 2 just before death. The pathological changes were similar
to the natural VHDV infection. Numerous virions accumulated adjacent to the nuclei
and a few were observed in the nuclei themselves, in thin sections of cultured cells
of the tenth passage (Fig. 6). Susceptible rabbits, inoculated with cell culture of the
tenth or fifteenth passages inactivated by 0.35% formaldehyde, could produce HI serum
antibodies to titres of 2 -211 at day 10 post inoculation and could resist the challenge
of virulent V H D V at day 15 post inoculation.
2
6
6
14
12
17
9
FIG.
5
Cytopathic effect (CPE) produced in a monolayer of DJRK cell culture by VHDV
Rounding of cells and intercellular bridges can be observed
____
• _
12126 - 1
7 | y y 9K»
X46K
FIG. 6
Thin section of a monolayer of DJRK cells infected by VHDV,
x 80,000
Virions aggregate adjacent to the disappearing nuclear membrane;
a few are seen in the nucleus
404
EPIDEMIOLOGY
Host range
It is probable that only rabbits are susceptible to V H D . N o evidence indicates
that horses, mules, donkeys, cattle, buffalo, sheep, goats, swine, dogs, cats or fowls
can be affected by the disease, though these animals have frequent contact with
infected rabbits and rabbit carcasses, excretions and virus-contaminated materials
in backyard rabbitries (5, 13, 16). M a n is not susceptible to V H D V , as evidenced
by the lack of disease in the millions of people who consume rabbit meat. N o case
of infection has been reported from large numbers of farmers, veterinarians and
laboratory technicians who come in contact with V H D V daily.
Attempts to infect laboratory rodents and fowls have all been unsuccessful. It
has been reported that white mice, rats (white, black, grey and brown), golden
hamsters and guinea pigs have been inoculated subcutaneously, intramuscularly,
peritoneally or orally with liver emulsions of infected rabbits. N o symptom has
manifested in these animals and H I antibodies cannot be detected in their sera (16).
A captive hare with pathological changes similar to V H D was found in 1986. Its
liver emulsion was inoculated into rabbits and typical V H D resulted. The H A titre
of that liver sample was 10 x 2 , which could be inhibited by V H D V antiserum.
Ninety-nine serum samples taken from hares had titres lower than l:21-l:2
(40 hares), 1:2 and 1:2 -1:2 (25 hares), and 1:2 -1:2 (9 hares). Hunters have never
seen any hares either infected or killed by a disease resembling V H D .
6
3
4
5
7
8
9
Susceptibility of rabbits
Domestic rabbits of all breeds appear to be fully susceptible. W h e n A n g o r a , Rex,
Chinchilla, Japanese White and fur hybrids are inoculated experimentally with virus
preparation, n o significant difference in morbidity and mortality is observed. In
natural outbreaks, however, wool rabbits may be more susceptible t h a n meat and
fur rabbits (16).
Susceptibility is clearly influenced by age. Rabbits less t h a n one m o n t h of age
cannot be infected clinically, and those between one and two months of age may be
infected at low percentages. The majority of infections are found in rabbits older
t h a n three m o n t h s . There is n o difference in susceptibility between male and female
adult rabbits (13).
Serum HI antibodies may exert effects on susceptibility. W h e n the antibody level
is above 2 , rabbits are usually resistant to experimental challenge (12). Forty-six
samples of rabbit serum collected in Jiangsu Province from 1975 to 1983 were
examined for H I antibodies to V H D . Titres of 44 samples were below 2 ; those of
only two samples reached 2 . This implies that V H D did not exist in Jiangsu province
before 1984, but the source of antibodies is not clear. Naturally resistant rabbit flocks
are not uncommon, even in isolated villages where V H D never occurs and vaccination
is not carried out (1).
4
2
4
Transmission
The V H D virus is present in blood, organs, secretions, excretions and on skin
and mucous membranes, principally during the late stage of infection and after death.
405
Direct or indirect contact with infected rabbits or carcasses, excretions and
contaminated objects m a y transmit infection to susceptible animals.
Rabbit wool pedlars, who shear and purchase rabbit wool even from infected
rabbits and carcasses, may play a significant role in the spread of the disease in the
People's Republic of China. They come to villages by coaches and trains and the
virus can be carried extensive distances on their h a n d s , clothes, shoes and scissors.
Through ignorance, farmers may carry the virus to their neighbours and relatives (19).
Infected rabbits m a y accidentally be mixed with healthy ones in rabbit processing
plants, especially during the incubation period. The virus is difficult to detect in meat;
however, by using E L I S A , the author has detected it in bone m a r r o w after viral
antigens h a d been concentrated from samples (19).
Since 1986, new outbreaks have occasionally occurred in healthy rabbitries after
the introduction of breeding rabbits from enzootic areas where vaccination with
inactivated vaccine had been regularly carried out. Young rabbits and vaccinated adults
may appear to be healthy when, in fact, they are subclinically infected (19).
Season
V H D is epizootic t h r o u g h o u t the year, without marked seasonal differences,
though outbreaks are comparatively more frequent during colder m o n t h s .
P R E S E N T S T A T U S OF V I R A L H A E M O R R H A G I C D I S E A S E
OF RABBITS IN T H E PEOPLE'S REPUBLIC OF C H I N A
The first case of viral haemorrhagic disease, involving an Angora rabbit imported
a few days before from the G e r m a n Democratic Republic, appeared in the spring
of 1984 in Wuxi City, Jiangsu Province, about 100 km from Shanghai. Except for
the young and suckling animals, almost all the rabbits in the affected rabbitry h a d
died of infection within a week. Pathogenic bacteria were not isolated and antibiotics
were ineffective; no infectious disease with similar symptoms and pathological changes
could be found by referring to the worldwide literature. In less than nine m o n t h s ,
the disease h a d spread rapidly to an area of approximately 50,000 k m , nearly
destroying most of the rabbitries within that area. At first, local scientists felt helpless
to control this horrible disease; before long, however, an inactivated vaccine was
developed which induced solid and effective immunity in rabbits. By the end of 1986
the disease h a d gradually been brought under control, and there is now hope of
eradication within a few years. However, the eradication p r o g r a m m e is not always
carried out as conscientiously as required by the veterinary authorities. Sporadic
outbreaks, usually limited to a single rabbitry or a small village, are still found in
other provinces, where vaccinations are not performed with the necessary vigour.
The disease readily subsides after all the rabbits of infected rabbitries are slaughtered
and the rabbits of the neighbouring rabbitries are vaccinated. The source of infection
probably remains rabbits introduced for breeding purposes, even if they appear to
be normal. These rabbits are not usually infected in an outbreak because they have
been vaccinated, and high levels of H I antibodies are detected in their sera.
2
406
LA MALADIE HÉMORRAGIQUE VIRALE DU LAPIN EN RÉPUBLIQUE POPULAIRE
DE CHINE : ÉPIDÉMIOLOGIE ET CARACTÉRISATION DU VIRUS. - Wei-Yan Xu.
Résumé: Récemment reconnue, la maladie hémorragique virale (VHD) est une
grave maladie infectieuse des lapins, qui entraîne des taux de morbidité et de
mortalité très élevés. Elle peut se déclarer à n'importe quelle période de l'année
et n'atteint que les lapins adultes à l'exclusion de tout autre animal domestique,
volaille ou rongeur de laboratoire. Elle se transmet par voie horizontale, par
contact direct ou indirect, et par toutes les voies. Il n'a pas été rapporté de cas
d'infection congénitale ni de transmission par vecteurs vivants.
Le virus responsable de la maladie hémorragique virale (VHDV) est présent
dans tous les tissus infectés, ainsi que dans les excrétions et sécrétions. Il s'agit
d'un virus icosaédrique nonenveloppé, proche des parvovirus. Les méthodes
classiques d'analyse du génome, telles que la Chromatographie haute
performance et la synthèse in vitro de l'ADN double brin, ont permis de
déterminer que le génome du VHDV était un ADN linéaire à un seul brin.
L'auteur a observé que le VHDV agglutine à un très haut titre les globules rouges
humains, quel que soit le groupe sanguin. Le virus reste stable en présence de
nombreux agents physiques et chimiques. Le VHDV a été adapté en culture
sur des souches cellulaires de rein de lapin (DJRK) où il entraîne un effet
cytopathogène. La culture cellulaire inactivée peut protéger les lapins inoculés
contre une souche virulente de VHDV.
La maladie est à présent bien contrôlée en République populaire de Chine ;
elle n'a cependant pas encore été totalement éradiquée.
MOTS-CLÉS : ADN à un brin - Chromatographie haute performance - Clonage
du génome - Culture cellulaire - Epidémiologie - Hémagglutination - Maladie
hémorragique virale du lapin - Parvovirus - Transmission - Virus de la maladie
hémorragique virale du lapin.
*
* *
LA ENFERMEDAD HEMORRÁGICA VIRAL DEL CONEJO EN LA REPÚBLICA
POPULAR DE CHINA: EPIDEMIOLOGÍA Y CARACTERIZACIÓN DEL VIRUS. Wei-Yan Xu.
Resumen: La enfermedad hemorrágica viral del conejo (VHD) es una
enfermedad recientemente identificada que ocasiona una morbilidad y una
mortalidad importantes. La enfermedad puede declararse en cualquier período
del año; sólo afecta los conejos adultos, sin tocar ningún otro animal, sea éste
doméstico o de laboratorio. Se transmite horizontalmente a través de todas las
vías de transmisión por contacto directo o indirecto. No existe evidencia de
infecciones congenitales ni ocasionadas por medio de vectores biológicos.
El virus responsable de la enfermedad hemorrágica viral (VHDV) está
presente en todos los tejidos, así como en las excreciones y secreciones. Se trata
de un virus semejante a los parvovirus, de forma icosaédrica y sin envoltura.
Métodos clásicos de análisis del genoma, como la cromatografía altamente
performante o la síntesis in vitro del ADN doble fila, determinaron que el
genoma del VHDV es un ADN linear de una fila. El VHDV puede aglutinar
eritocitos humanos, alcanzando títulos muy elevados, sin distinción entre los
grupos sanguíneos. Presenta una buena resistencia a muchos factores físicos
y químicos. El VHDV ha sido adaptado en cultivos celulares obtenidos a partir
de cepas de riñon de conejo (DJRK), produciendo un efecto citopatogénico.
407
El cultivo celular inactivado puede proteger los conejos inoculados contra una
cepa virulenta de VHDV.
Actualmente, la enfermedad está bajo control en la República Popular de
China, a pesar de no haberse logrado todavía su total erradicación.
PALABRAS CLAVE: ADN de una fila - Clonación del genoma Cromatografía altamente performante - Cultivo celular - Enfermedad
hemorrágica viral del conejo - Epidemiología - Hemaglutinación - Parvovirus Transmisión - Virus de la enfermedad hemorrágica viral del conejo.
*
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